1. Field of the Invention:
The present invention relates to a quartz crystal oscillator that packages a quartz crystal blank and an IC (integrated circuit) chip, which integrates an oscillator circuit using the crystal blank, into a container having a recess in each of both main surfaces. In particular, the present invention relates to a crystal oscillator with improved tolerance to radiation such as X-rays, and a method of manufacturing the same.
2. Description of the Related Art:
The crystal oscillator configured by packaging the crystal blank and the IC chip integrating the oscillator circuit using the crystal blank into a container can easily adopt a configuration of a surface mount component, and is widely built-in as a reference source of frequency and time in a mobile electronic device, for example, typified by a mobile phone.
There are various configurations housing the crystal blank and the IC chip in the crystal oscillator. One configuration among them employs a container body that has a recess on each of both main surfaces and is formed to have an H-shape in a sectional view, and separately accommodates the IC chip and the crystal blank in the recesses. FIG. 1A is a sectional view showing an example of a configuration of such a crystal oscillator in a related art. FIG. 1B is a bottom view of the crystal oscillator shown in FIG. 1A.
The crystal oscillator shown in FIGS. 1A and 1B has a surface mount configuration suitable for surface-mounting on a circuit board or a wiring board, and includes container body 1 having recesses 1a and 1b in both respective main surfaces. Crystal blank 2 is accommodated in one recess 1a of container body 1, and IC chip 3 is accommodated in the other recess 1b. Mounting electrodes 11, which are used for surface-mounting the crystal oscillator on a circuit board or the like, are provided at four corners of the end surface surrounding recess 1b in container body 1. Mounting electrode 11 includes, for example, a power supply terminal, a ground terminal, and an oscillation output terminal.
Crystal blank 2 is, for example, an AT-cut quartz crystal blank having a substantially rectangular shape, and includes an excitation electrode (not shown) on each of both main surfaces. Extending electrodes extend from the excitation electrodes toward both sides of one end of crystal blank 2, respectively. Both the sides of the one end of crystal blank 2, to which the respective extending electrodes extend, are fixed to crystal holding terminal 4 provided on the bottom of one recess 1a of container body 1 by conductive adhesive 5. Metal ring 6 is provided at an end surface of container body 1 surrounding recess 1a. Metal cover 7 is bonded with metal ring 6 by seam welding, thereby hermetically encapsulating crystal blank 2 in recess 1a. 
IC chip 3 is formed by integrating electronic circuits including an oscillator circuit using crystal blank 2 on a semiconductor substrate, and accommodated in the other recess 1b of container body 1. A silicon substrate is widely employed as a semiconductor substrate. These electronic circuits are formed on one main surface of the semiconductor substrate constituting IC chip 3. This main surface is referred to as a circuit formation plane. IC terminals 8 for electrical connection to the electronic circuits in IC chip 3 from the outside are provided on the circuit formation plane of IC chip 3. IC terminals 8 include, for example, a pair of crystal connecting terminals used for electrical connection to crystal blank 2, a power supply terminal, a ground terminal, and an output terminal.
Connection electrodes 9 are provided on the bottom surface of recess 1b according to IC terminals 8. IC chip 3 is fixed to the bottom surface of recess 1b by flip-chip bonding using bumps 10. At this time, IC terminals 8 and corresponding connection electrodes 9 are electrically and mechanically connected to each other by bumps 10. Connection electrodes 9 corresponding to the crystal connecting terminals among IC terminals 8 are electrically connected to crystal holding terminals 4 on the bottom surface of recess 1a by conductive paths that are formed in container body 1 including a via holes (not shown). Accordingly, crystal blank 2 is electrically connected to the oscillator circuit inside IC chip 3. Connection electrodes 9 corresponding to the power supply terminal, the ground terminal, the oscillation output terminal and the like among IC terminals 8 are electrically connected to corresponding mounting electrodes 11 via conductive paths provided in container body 1.
The electronic circuit integrated into IC chip 3 in the crystal oscillator is not limited to the oscillator circuit. For example, an electronic circuit such as a temperature compensating circuit can be integrated into IC chip 3; the temperature compensating circuit compensates temperature-frequency characteristics of crystal blank 2 and can acquire a constant oscillation frequency irrespective of the ambient temperature. A crystal oscillator capable of highly accurately maintaining the oscillation frequency against temperature variation by incorporating a temperature compensating circuit is referred to as a temperature compensated crystal oscillator (TCXO).
In recent years, as typified by airport security screening, X-ray screening using relatively low intensity X-rays has widely been used. In general, in a semiconductor integrated circuit, as described in Section 5.9 “Application of Radiation” in “Technology on MOS Circuit Design, Manufacturing and Reliability”, Hidenori Oyama, Masakazu Nakabayashi, Kiyoteru Hayama, and Kei Eguchi, Morikitashuppan, April 2008 (ISBN 978-4-627-77381-3), it is known that when ionizing radiation, such as X-rays and γ-rays, is incident, device characteristics may vary. Repeated application of radiation accumulates variation. This accumulation of variation is referred to as a total dose effect. In the crystal oscillator, application of X-rays slightly changes the oscillation frequency. In a case where the crystal oscillator is a TCXO, the frequency accuracy is originally high. Accordingly, even X-ray application in low intensity X-ray screening can cause a problem of frequency change owing to the application of radiation.
Conventionally, methods of protection against radiation in electronic devices include, for example, a method of manufacturing a semiconductor integrated circuit by a special fabrication process or a method of housing the entire electronic device in a radiation shield. However, these methods are for high energy or high dose radiation, and have a large-scale configuration and significantly increase manufacturing cost. These methods are not necessarily optimal as measures against low intensity X-ray application in airport security screening and the like.
Measures against electromagnetic noise in an extent of wireless frequencies at an individual component level or part level include, for example, those described in JP2001-319987A and US2001/0045626. However, radiation such as X-rays and electromagnetic noise with frequencies of several hundred gigahertz at the highest are completely different from each other in physical properties, especially in fundamental principle of effect to be exerted on the electronic circuit. The techniques described in JP2001-319987A and US 2001/0045626 cannot be utilized in measures against application of ionizing radiation as they are.
As described above, X-ray screening using relatively low energy and low dose X-rays, such as thorough airport security screening, has widely been conducted in recent years. Along therewith, particularly in a crystal oscillator that outputs highly accurate oscillation frequencies, such as a TCXO, there have been concerns about frequency change along with X-ray application.